Patent Application
20070022756
This disclosure relates generally to control systems for turbines, and more specifically to measuring generator operating parameters to detect a loss of load that results in a large unbalance between power and load that would result in turbine-generator overspeed trip and provide turbine overspeed protection that will avoid a turbine trip due to the emergency overspeed trip device.
In a typical reheat turbine speed-load control system, valves regulate steam flow to both the high-pressure and reheat stages of the turbine in order to maintain the turbine speed at a predetermined reference speed. A main control valve regulates steam flow into the high-pressure stages and an intercept control valve regulates steam flow into the reheat stages. On the occurrence of an overspeed condition while the generator is separated from the system, the main control valve and intercept control valve are totally or partially closed to decrease steam flow into their respective stages to effect a reduction of turbine speed to the reference speed.
These reheat turbine speed-load control systems operate effectively to prevent an emergency overspeed trip on the occurrence of sudden total load loss where the load lost is small compared to the total generating capacity of the turbine-generator set. In this case a rapid complete closure of the intercept control valve causes the loss of a major portion of the driving power normally provided by the reheat stages and similarly steam flow to the high-pressure stages is reduced by closure of the main control valve. The control basis for these valve closures is the difference between the actual speed and the reference speed.
On the occurrence of sudden severe total loss of load where the load lost is comparable to a significant portion of the generating capacity of the turbine generator set, the turbine speed-load control systems, as described above, can not prevent an emergency overspeed trip. Delaying rapid complete closure of the intercept valve and control valve until the actual speed increases above the reference speed in order to avoid an emergency overspeed trip is not effective due to the larger control and intercept valve opening and the larger power density at high load conditions. Initiation of control valve and intercept valve closure must be advanced in time sufficiently to begin cut-off of steam prior to actual speed increase. Total electrical load loss is characteristically a rapid event when compared to rate of change for turbine power production. This situation can be used to provide detection of the conditions leading to turbine generator overspeed as described in this disclosure and prior art. When the characteristic conditions of load loss and unbalance between power and load are detected as described in this disclosure, rapid closure of turbine steam control valve and intercept valve is initiated, removing a major portion of the driving power to avoid an emergency overspeed trip. Hence, once the steam supply to the turbine is cut-off, the speed will increase due to the entrained steam mass as it proceeds to exhaust, but the speed increase will not be as rapid and will peak at a lower value thus avoiding an emergency overspeed trip. After the entrained steam is exhausted, speed will decrease and the turbine intercept valve and control valve will open based on speed-load control.
Disclosed herein is a method for controlling a turbine-generator including: detecting an unbalance between turbine mechanical power and generator electrical load; measuring a duration of the power-load unbalance; measuring a rate of loss for an electrical load; and regulating steam flow through said turbine responsive to the rate of loss for an electrical load and the duration of said power-load unbalance.
Further disclosed herein is a turbine-generator control system including: a power-load unbalance module; a turbine speed-load control module; and one or more close override devices wherein the power-load unbalance module detects a power-load unbalance between a turbine and a generator and identifies a cause of the power-load unbalance.
Yet further disclosed herein is a system for anticipating and limiting overspeed of a turbine including: means for improved detection of a power-load unbalance; means for determining the duration of the power-load unbalance; means for calculating a current rate of change; and means for regulating steam flow through the turbine being responsive to at least one of the means for detecting a power-load unbalance, the means for determining the duration of the power-load unbalance, and the means for calculating a current rate of change.
Other systems, methods, and/or computer program products according to exemplary embodiments will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional systems, methods, and/or computer program products be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.
These and other features, aspects, and advantages of the disclosure hereof will become better understood when the following detailed description is read with reference to the accompanying figures, wherein:
The turbine control system 30 includes a turbine speed-load control module 32, a power-load unbalance module 34, and one or more fast close override modules 36. The turbine speed-load control module 32 controls the position of the main steam stop valve 50, the inlet control valve 52, the reheat intercept valve 54, and the reheat stop valve 56. The power-load unbalance module 34 detects a stage pressure of the turbine, illustrated in
Turning now to
The time delay pick up 116 is programmed to output a power-load unbalance out of limit signal 120 which is a logic signal of true when the input to the time delay pick up 116 is true and remains true for at least ten milliseconds. Otherwise, the power-load unbalance out of limits signal 120 remains at a logic level of false. The minimum sustained power unbalance duration is verified by the time delay pick up 116. The input to the time delay pick up 116 must be held true for ten milliseconds before the output of the time delay pick up 116 is set to true. If the input to the time delay pick up 116 becomes false before the end of ten milliseconds then the ten millisecond timer of the time delay pick up 116 is reset to zero and must again be held true for a minimum of ten milliseconds. At any time, if the input to the time delay pick up 116 goes to a logic level of false, the power-load unbalance out of limits signal 120 will go to a logic level of false. It will be appreciated that the duration of time required for the time delay pick up 116 can be set to any suitable time and that the time of ten milliseconds as used, based on the transient electrical load characteristics detected by sensing devices 98, herein is the amount of time used in an exemplary embodiment of the time delay pick up 116.
Continuing with
The turbine control system 30 also includes a AND gate 124, an OR gate 126, a master set-reset logic element 136, a secondary set-reset logic element 128, and one or more time delay pick up elements 130, 132. It is necessary for both the power-load unbalance current rate out of limit signal 122 and the power-load unbalance out of limit signal 120 to be simultaneously true in order to initiate and latch a power-load unbalance event, this condition is determined by the AND gate 124. When the power-load unbalance out of limit signal 120 becomes false it initiates a reset signal to the master set-reset latch element 136, which clears the power-load unbalance event. In an exemplary embodiment of the reheat turbine-generator control system 30, an optional programmable delay time may be added before a power-load unbalance event is initiated. The optional programmable delay time is controlled by the OR gate 126, the secondary set-reset latch element 128, and the time delay pick up elements 130, 132. The power-load unbalance event will use the optional programmable delay time only if power-load unbalance delay enabled signal 134 has a logical value of true.
Turning now to
In order to make the above calculation the per unit current 107 at time t1 and the per unit current 107 at time t2 must be determined. In an exemplary embodiment, the per unit current 107 is sampled at 500 Hz and stored in a circular list 208, which is used in calculating the per unit current rate of change 226.
Continuing with
The rate discriminator filter 206 has the desired characteristic for detecting an actionable current rate of change when used with rectified current inputs. When the output of the rate discriminator filter 206 drops below a set threshold, a large current drop is indicated. After detecting a large current drop, an initial time tag (t1) 214 is then stored and the sampled output of the discriminator rate filter 206 is stored in a buffer 222. The set threshold level of the discriminator rate filter 206 is based on the nominal per unit current operating point of the turbine-generator system 202.
Additionally, when the power unbalance logic signal 200 is a logical true the output of the discriminator rate filter 206, which is stored in the buffer 222, is scanned to find a minimum and the corresponding time tag (t2) 216. If a minimum is not achieved in the ten millisecond time frame then the current measured at ten milliseconds is used as the minimum and time tag (t2) 216 is thereby established. With the time tag (t1) 214 and time tag (t2) 216 established, the per unit current stored in the circular list 208 can be then accessed for the computation of the current rate of change 226. As depicted in
A power-load unbalance event is triggered when both the power-load unbalance current rate out of limit signal 122 and the power-load unbalance out of limit signal 120 are true. Once a power-load unbalance event is triggered the reheat turbine-generator control system 30 actuates at least one of the main steam stop valve 50, the inlet control valve 52, the reheat intercept valve 54, and the reheat stop valve 56. The fast close override devices 36 may be used to cause the inlet control valve 52 and the reheat intercept valve 54 to rapidly close and cut off steam to the steam turbine resulting in turbine power reduction. Additionally, the reopening of the inlet control valve 52 and the reheat intercept valve 54 is coordinated by both the power-load unbalance module 34 and the speed load control module 32.
While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
